linux-ext4 - Re: [PATCH V3] fix bb_prealloc_list corruption due to wrong group locking

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Date:	Mon, 16 Mar 2009 12:48:16 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Frank Mayhar <fmayhar@...gle.com>
CC:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH V3] fix bb_prealloc_list corruption due to wrong group
 locking

Frank Mayhar wrote:
> On Mon, 2009-03-16 at 12:28 -0500, Eric Sandeen wrote:
>> This is for Red Hat bug 490026,
>> EXT4 panic, list corruption in ext4_mb_new_inode_pa
>>
>> ext4_lock_group(sb, group) is supposed to protect this list for
>> each group, and a common code flow to remove an album is like
>> this:
>>
>>     ext4_get_group_no_and_offset(sb, pa->pa_pstart, &grp, NULL);
>>     ext4_lock_group(sb, grp);
>>     list_del(&pa->pa_group_list);
>>     ext4_unlock_group(sb, grp);
>>
>> so it's critical that we get the right group number back for
>> this prealloc context, to lock the right group (the one 
>> associated with this pa) and prevent concurrent list manipulation.
> 
> Eric, this may just be coincidence, but is it possible that this may be
> related to our bitmap problem I described last week?  We haven't tracked
> it down yet but it certainly smells like a race and your fix corrects
> just such a race in the same code.
> 
> The bitmap problem, btw, involves stuff apparently being marked as used
> when it's really free (or something very much like that), ultimately
> resulting in double frees.

Hi Frank - I don't *think* so just because deleted items are poisoned
and I would expect that we'd trip over a bad pointer in the corrupted
list item as the first indicator of trouble... but I could be wrong.

I think you said you could reproduce it, right?  So certainly worth
testing with this fix I suppose.

Thanks,
-Eric
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